n6 The Realm of Nature CHAP. 



from a cloudless sky. Rain more often originates in clouds. 

 The upper part of a very deep layer of cloud is less dust- 

 laden than the lower ; the motes accordingly form larger 

 water-drops, and these descend comparatively quickly, 

 overtaking and embodying smaller globules as they fall, 

 until they emerge from the cloud as large drops of water. 

 If the cloud floats very high above warm air, the vapour of 

 which is unsaturated, the raindrops will evaporate as they 

 fall and may vanish before reaching the Earth. But if the 

 cloud is low or the vapour in the air traversed by the rain- 

 drops is nearly or quite saturated, there is so little evapora- 

 tion that they reach the surface undiminished or even in- 

 creased in size. When much water-vapour is rapidly con- 

 densed near the surface or over air which is fully charged 

 with vapour, there must be a great fall of rain. Herice, 

 when a hot vapour-laden sea-wind blows against the side of 

 a mountain, the air rises, and growing cold in consequence 

 ( 163), the dew-point is reached and passed, and deluges 

 of rain fall, while dark masses of clouds fill the sky. On 

 the other hand, when wind blows over a mountain range 

 and descends on the other side, it grows warmer as it sinks, 

 evaporates all the cloud it carries, and becomes a drying 

 wind upon the low ground. Rainfall is measured by the 

 rain gauge, and its amount is stated in the number of 

 inches of water which would accumulate on a level surface 

 if the rain of a year were to rest where it fell. 



1 70. Snow is produced when water-vapour condenses at 

 a temperature below the freezing-point. The water forms 

 small clear spicules of ice which always cross at an angle 

 of 60, so that snow-crystals usually have six rays uniformly 

 arranged about a centre ; but the variety of forms is very 

 great. A number of crystals getting hooked or felted 

 together form a snow-flake, and the fluttering showers of 

 flakes rest lightly on the ground, sometimes covering it to 

 the depth of several feet. One foot of snow is, roughly 

 speaking, equivalent to one inch of rain. The whiteness of 

 snow is produced by the reflection and refraction of light 

 again and again amongst the numerous small crystals. 

 The real colour is bluish or greenish like a block of ice. 



